PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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option that can be eliminated. The choice of material is driven by availability and other mechanical requirements thus leading to the elimination of tensile stress as the best operative option to ensure that a system will not suffer CISCC. Laser peening has been identified as a technical solution that provides a compressive stress deeper than minimally required and a technology that can be efficiently applied to large structures with high reliability and quality assurance thus providing an economic solution to extend the field life of the MPCs.

3. Test program

In our work we undertook a test program to demonstrate that laser peening was capable of significantly extending the MPC design life. The laser peening is well suited to prevent CISCC in 316L material because of its ability as shown by data herein, to develop very deep (> 5mm) depths of compressive stress. The program centered on using relatively large,11 by 11 inch, panels of 316L stainless steel of thickness 0.65 inch for peening and residual stress measurements and smaller 6 x 5 x 0.65 inch panels for CISCC testing. The panels were welded mid-section with process and technique directly traceable to that used by Holtec International for canister fabrication. Susceptibility to CISCC was evaluated using the ASTM G36-94 (2013) test procedure [10]. Panels were laser peened over half their area with the other half left in an unpeened as-welded state thereby enabling simultaneous exposure and testing of the peened vs. unpeened areas. Additional panels, some welded only and others welded and laser peened, were measured for residual stress by Hill Engineering using the contour method [11,12]. Finally to conclusively relate residual stress measurements taken in test panels to the stresses expected in dry canisters, we performed a finite element analysis that related the stress measured in the test panels which had edge boundaries free to strain to that to be found in the constrained geometry of the full canisters. 4. Laser peening and residual stresses Peening is a process that plastically compresses surface material resulting in transverse (Poisson’s) expansion [13,14,15]. A thicker component’s ability to resist the transverse straining results in a local buildup of compressive stress. For thinner components, the peening results in strain and shape change. Such is the case for all types of compressive surface treatments including shot, laser, and ultrasonic peening and non-impact processes such as deep cold rolling. Processes such as shot peening and ultrasonic needle peening induce forces that are Hertzian in nature, generating transverse as well as normal strains and resulting in significant cold work. Laser peening, in contrast, employs high pressure plasma that generates fully normal compressive force. Figure 1 illustrates how the process works for laser peening keeping in mind that the concept of plastic compression and transverse expansion is common to all treatments. In the LP process, nanosecond scale, intense laser pulses create a plasma in a confined geometry as shown in Figure 1 and thereby generate pressure pulses sufficiently intense to locally plastically deform the near surface of the metal being treated. An ablative layer can be applied to the surface being peened to act as the source of material for the plasma. In other work this layer is omitted resulting in only a very shallow (10 to 20 µm thick) layer of recast material that can be left on the surface or easily polished off. Use of a water tamper increases the generating pressure by an order of magnitude thus making the process more efficient [16]. Detailed results of the peening of a specific component depend on its material and geometry, existing residual compressive stresses, and desired strains and microstructure changes. With the laser peening process these modifications to stress state and/or shape and microstructure can be precisely generated in parts in a spot-by-spot manner. Laser peened materials typically demonstrate higher resistance to cracking and corrosion and are becoming widely used in manufacturing.